As is generally known, the proportion of fresh air in the cylinders of an internal combustion engine is an important process variable. To carry out combustion of fuel in a manner which is as effective as possible with the lowest amount of pollution possible, a precise adjustment of the proportion of fuel and of the proportion of fresh air in the cylinders is particularly significant. As is generally known, to determine the proportion of fresh air in the cylinders of an internal combustion engine, it is possible to measure the pressure in the intake system, it being assumed that at the moment when the inlet valves are closed, the pressure in the intake system is consistent with the pressure in the cylinders. As is also generally known, to determine the proportion of fresh air in the cylinders of an internal combustion engine, the mass flow of fresh air which flows into the intake system can be measured, for example by a hot film air mass meter. The mentioned methods cannot determine the manner in which the fresh air is distributed onto the individual cylinders of an internal combustion engine. The mentioned methods also suffer from the shortcoming that, inter alia, due to the storage effect of the intake system and to the inertia of the fresh air, the intake system or the cylinders are filled and emptied in a delayed manner and thus the signals used respectively in dynamic operating phases of the internal combustion engine do not describe the actual proportion of fresh air in the cylinders. However, for a low-pollution conversion of fuel, it is absolutely necessary to know as accurately as possible the proportion of fresh air which is contained in the individual cylinders. According to DE 10 2007 013 460 A1, it is already known to use pressure sensors to determine the pressure in the cylinders of an internal combustion engine. The combustion phase position and/or the indicated mean effective pressure are determined by means of the cylinder pressure. The combustion phase position and/or the indicated mean effective pressure are used in conjunction with a known thermal efficiency to determine an energy conversion and/or a proportion of fuel to be fed to the cylinders. Furthermore, the air mass in the cylinders of the internal combustion engine is determined from the proportion of fuel to be fed to the cylinders, in conjunction with a combustion air ratio which is established using a residual oxygen sensor, and with the known oxygen concentration in the air. In other words, a distribution of the fresh air onto the individual cylinders of an internal combustion engine can be determined by this method. However, this requires cylinder pressure sensors which, as is known, are expensive and nondurable, that is to say, hitherto they have not found their way into series production. Furthermore, in this method, a residual oxygen sensor is required, so that differences in measurements which may result from the two sensors which are used become more frequent. DE 10 2008 023 104 A1 also discloses a method for starting an internal combustion engine, the method comprising the determination of a cylinder stroke during the starting of the engine as a reaction to a distributing pipe pressure. DE 102 36 615 A1 discloses a method for detecting a signal representing the pressure in the combustion chamber of an internal combustion engine operating with direct injection, the fuel injection valves of which are provided for actuation with a piezoelectric converter element. The signal which is picked up by the piezoelectric converter element can be used as a pressure signal. The signal picked up outside the injection phase is advantageously used to detect the knocking signal.